chore(datasets): trim sampler comment and drop duplicate tests

Remove the verbose dataloader-guard comment and the two EpisodeAwareSampler tests
that duplicated existing validation/warning coverage (no coverage loss).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lerobot/common/train_utils.py

@@ -49,8 +49,19 @@ def get_step_checkpoint_dir(output_dir: Path, total_steps: int, step: int) -> Pa
     return output_dir / CHECKPOINTS_DIR / step_identifier
 
 
-def save_training_step(step: int, save_dir: Path) -> None:
-    write_json({"step": step}, save_dir / TRAINING_STEP)
+def save_training_step(
+    step: int, save_dir: Path, num_processes: int | None = None, batch_size: int | None = None
+) -> None:
+    state: dict = {"step": step}
+    # num_processes and batch_size are recorded so a resumed run can detect a changed world size or
+    # batch size: the sampler's resume offset is computed from the (num_processes, batch_size) that
+    # produced `step`, since both scale how many sampler positions a step consumes (see
+    # compute_sampler_state).
+    if num_processes is not None:
+        state["num_processes"] = num_processes
+    if batch_size is not None:
+        state["batch_size"] = batch_size
+    write_json(state, save_dir / TRAINING_STEP)
 
 
 def load_training_step(save_dir: Path) -> int:
@@ -58,6 +69,16 @@ def load_training_step(save_dir: Path) -> int:
     return training_step["step"]
 
 
+def load_training_num_processes(checkpoint_dir: Path) -> int | None:
+    """World size recorded at checkpoint time, or None for checkpoints written before it was stored."""
+    return load_json(checkpoint_dir / TRAINING_STATE_DIR / TRAINING_STEP).get("num_processes")
+
+
+def load_training_batch_size(checkpoint_dir: Path) -> int | None:
+    """Per-process batch size recorded at checkpoint time, or None for older checkpoints."""
+    return load_json(checkpoint_dir / TRAINING_STATE_DIR / TRAINING_STEP).get("batch_size")
+
+
 def update_last_checkpoint(checkpoint_dir: Path) -> Path:
     last_checkpoint_dir = checkpoint_dir.parent / LAST_CHECKPOINT_LINK
     if last_checkpoint_dir.is_symlink():
@@ -75,6 +96,8 @@ def save_checkpoint(
     scheduler: LRScheduler | None = None,
     preprocessor: PolicyProcessorPipeline | None = None,
     postprocessor: PolicyProcessorPipeline | None = None,
+    num_processes: int | None = None,
+    batch_size: int | None = None,
 ) -> None:
     """This function creates the following directory structure:
 
@@ -100,6 +123,10 @@ def save_checkpoint(
         scheduler (LRScheduler | None, optional): The scheduler to save the state from. Defaults to None.
         preprocessor: The preprocessor/pipeline to save. Defaults to None.
         postprocessor: The postprocessor/pipeline to save. Defaults to None.
+        num_processes (int | None, optional): Distributed world size to record for sample-exact
+            resume. Defaults to None (not recorded).
+        batch_size (int | None, optional): Per-process batch size to record for sample-exact
+            resume. Defaults to None (not recorded).
     """
     pretrained_dir = checkpoint_dir / PRETRAINED_MODEL_DIR
     policy.save_pretrained(pretrained_dir)
@@ -112,7 +139,9 @@ def save_checkpoint(
         preprocessor.save_pretrained(pretrained_dir)
     if postprocessor is not None:
         postprocessor.save_pretrained(pretrained_dir)
-    save_training_state(checkpoint_dir, step, optimizer, scheduler)
+    save_training_state(
+        checkpoint_dir, step, optimizer, scheduler, num_processes=num_processes, batch_size=batch_size
+    )
 
 
 def save_training_state(
@@ -120,6 +149,8 @@ def save_training_state(
     train_step: int,
     optimizer: Optimizer | None = None,
     scheduler: LRScheduler | None = None,
+    num_processes: int | None = None,
+    batch_size: int | None = None,
 ) -> None:
     """
     Saves the training step, optimizer state, scheduler state, and rng state.
@@ -131,10 +162,12 @@ def save_training_state(
             Defaults to None.
         scheduler (LRScheduler | None, optional): The scheduler from which to save the state_dict.
             Defaults to None.
+        num_processes (int | None, optional): Distributed world size to record. Defaults to None.
+        batch_size (int | None, optional): Per-process batch size to record. Defaults to None.
     """
     save_dir = checkpoint_dir / TRAINING_STATE_DIR
     save_dir.mkdir(parents=True, exist_ok=True)
-    save_training_step(train_step, save_dir)
+    save_training_step(train_step, save_dir, num_processes=num_processes, batch_size=batch_size)
     save_rng_state(save_dir)
     if optimizer is not None:
         save_optimizer_state(optimizer, save_dir)

src/lerobot/datasets/__init__.py

@@ -50,7 +50,7 @@ from .lerobot_dataset import LeRobotDataset
 from .multi_dataset import MultiLeRobotDataset
 from .pipeline_features import aggregate_pipeline_dataset_features, create_initial_features
 from .pyav_utils import check_video_encoder_parameters_pyav, detect_available_encoders_pyav
-from .sampler import EpisodeAwareSampler
+from .sampler import EpisodeAwareSampler, compute_sampler_state
 from .streaming_dataset import StreamingLeRobotDataset
 from .utils import DEFAULT_EPISODES_PATH, create_lerobot_dataset_card
 from .video_utils import VideoEncodingManager
@@ -82,6 +82,7 @@ __all__ = [
     "aggregate_stats",
     "convert_image_to_video_dataset",
     "create_initial_features",
+    "compute_sampler_state",
     "create_lerobot_dataset_card",
     "column_for_style",
     "delete_episodes",

src/lerobot/datasets/sampler.py

@@ -14,14 +14,36 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import math
 from collections.abc import Iterator
 
+import numpy as np
 import torch
 
 logger = logging.getLogger(__name__)
 
 
 class EpisodeAwareSampler:
+    """Sampler over episode frames that stores only per-episode boundaries.
+
+    Logical positions map to frame indices on the fly (O(num_episodes) construction memory)
+    instead of materializing a Python list of every frame index.
+
+    Each epoch is shuffled with a `torch.randperm` seeded from `(seed, epoch)`, so the data order
+    is a pure function of `(seed, epoch)`: it reproduces on every rank without synchronizing the
+    global RNG (no `generator` to sync across distributed ranks), and `state_dict` /
+    `load_state_dict` resume a run sample-exactly by regenerating the epoch's permutation and
+    continuing from the saved offset. Each call to `__iter__` advances the epoch. During a
+    resumed epoch, `__len__` still reports the full length.
+
+    Epoch advancement: `__iter__` eagerly advances the epoch, and `set_epoch` / `load_state_dict`
+    set it explicitly. Within a single run callers should rely on exactly one of these mechanisms,
+    not both: advancing the epoch by hand *and* letting `__iter__` auto-advance over the same
+    iterations would skip or repeat epochs. The training loop drives it purely through `__iter__`
+    (via `cycle`); `set_epoch` / `load_state_dict` are used only to (re)position before iteration
+    starts (e.g. on resume or in tests).
+    """
+
     def __init__(
         self,
         dataset_from_indices: list[int],
@@ -30,57 +52,125 @@ class EpisodeAwareSampler:
         drop_n_first_frames: int = 0,
         drop_n_last_frames: int = 0,
         shuffle: bool = False,
+        seed: int = 0,
     ):
-        """Sampler that optionally incorporates episode boundary information.
-
+        """
         Args:
-            dataset_from_indices: List of indices containing the start of each episode in the dataset.
-            dataset_to_indices: List of indices containing the end of each episode in the dataset.
-            episode_indices_to_use: List of episode indices to use. If None, all episodes are used.
-                                    Assumes that episodes are indexed from 0 to N-1.
-            drop_n_first_frames: Number of frames to drop from the start of each episode.
-            drop_n_last_frames: Number of frames to drop from the end of each episode.
+            dataset_from_indices: Start index of each episode in the dataset.
+            dataset_to_indices: End index of each episode in the dataset.
+            episode_indices_to_use: Episode indices to use; None means all.
+            drop_n_first_frames: Frames to drop from the start of each episode.
+            drop_n_last_frames: Frames to drop from the end of each episode.
             shuffle: Whether to shuffle the indices.
+            seed: Seed the permutation is derived from (together with the epoch).
         """
         if drop_n_first_frames < 0:
             raise ValueError(f"drop_n_first_frames must be >= 0, got {drop_n_first_frames}")
         if drop_n_last_frames < 0:
             raise ValueError(f"drop_n_last_frames must be >= 0, got {drop_n_last_frames}")
 
-        indices = []
-        for episode_idx, (start_index, end_index) in enumerate(
-            zip(dataset_from_indices, dataset_to_indices, strict=True)
-        ):
-            if episode_indices_to_use is None or episode_idx in episode_indices_to_use:
-                ep_length = end_index - start_index
-                if drop_n_first_frames + drop_n_last_frames >= ep_length:
-                    logger.warning(
-                        "Episode %d has %d frames but drop_n_first_frames=%d and "
-                        "drop_n_last_frames=%d removes all frames. Skipping.",
-                        episode_idx,
-                        ep_length,
-                        drop_n_first_frames,
-                        drop_n_last_frames,
-                    )
-                    continue
-                indices.extend(range(start_index + drop_n_first_frames, end_index - drop_n_last_frames))
+        from_indices = np.asarray(dataset_from_indices, dtype=np.int64)
+        to_indices = np.asarray(dataset_to_indices, dtype=np.int64)
+        if from_indices.shape != to_indices.shape:
+            raise ValueError(
+                f"dataset_from_indices and dataset_to_indices must have the same length, "
+                f"got {len(from_indices)} and {len(to_indices)}"
+            )
 
-        if not indices:
+        used = np.ones(len(from_indices), dtype=bool)
+        if episode_indices_to_use is not None:
+            used = np.zeros(len(from_indices), dtype=bool)
+            used[np.asarray(episode_indices_to_use, dtype=np.int64)] = True
+
+        starts = from_indices + drop_n_first_frames
+        lengths = to_indices - drop_n_last_frames - starts
+        for episode_idx in np.flatnonzero(used & (lengths <= 0)):
+            logger.warning(
+                "Episode %d has %d frames but drop_n_first_frames=%d and "
+                "drop_n_last_frames=%d removes all frames. Skipping.",
+                episode_idx,
+                to_indices[episode_idx] - from_indices[episode_idx],
+                drop_n_first_frames,
+                drop_n_last_frames,
+            )
+        used &= lengths > 0
+        if not used.any():
             raise ValueError(
                 "No valid frames remain after applying drop_n_first_frames and drop_n_last_frames. "
                 "All episodes were either filtered out or had too few frames."
             )
 
-        self.indices = indices
+        self._starts = starts[used]
+        self._cum_lengths = np.cumsum(lengths[used])
+        self._num_frames = int(self._cum_lengths[-1])
         self.shuffle = shuffle
+        self.seed = seed
+        self._epoch = 0
+        self._start_index = 0
+
+    @property
+    def indices(self) -> list[int]:
+        """Materialized frame indices in unshuffled order; O(num_frames), introspection only."""
+        return [self._frame_index(k) for k in range(self._num_frames)]
+
+    def set_epoch(self, epoch: int) -> None:
+        self._epoch = epoch
+
+    def state_dict(self) -> dict:
+        return {"epoch": self._epoch, "start_index": self._start_index}
+
+    def load_state_dict(self, state: dict) -> None:
+        self._epoch = state["epoch"]
+        self._start_index = state["start_index"]
+
+    def _epoch_generator(self, epoch: int) -> torch.Generator:
+        # Derive a per-epoch seed from (seed, epoch) so the permutation is a pure function of both
+        # and reproduces identically on every rank without touching the global RNG.
+        epoch_seed = int(np.random.SeedSequence([self.seed, epoch]).generate_state(1, dtype=np.uint64)[0])
+        return torch.Generator().manual_seed(epoch_seed)
+
+    def _frame_index(self, position: int) -> int:
+        episode = int(np.searchsorted(self._cum_lengths, position, side="right"))
+        position_in_episode = position - (int(self._cum_lengths[episode - 1]) if episode > 0 else 0)
+        return int(self._starts[episode]) + position_in_episode
 
     def __iter__(self) -> Iterator[int]:
+        # Advance epoch state eagerly, not on first consumption of the generator.
+        epoch, start = self._epoch, self._start_index
+        self._epoch += 1
+        self._start_index = 0
+        return self._iter_epoch(epoch, start)
+
+    def _iter_epoch(self, epoch: int, start: int) -> Iterator[int]:
         if self.shuffle:
-            for i in torch.randperm(len(self.indices)):
-                yield self.indices[i]
+            order = torch.randperm(self._num_frames, generator=self._epoch_generator(epoch))
+            for k in range(start, self._num_frames):
+                yield self._frame_index(int(order[k]))
         else:
-            for i in self.indices:
-                yield i
+            for k in range(start, self._num_frames):
+                yield self._frame_index(k)
 
     def __len__(self) -> int:
-        return len(self.indices)
+        return self._num_frames
+
+
+def compute_sampler_state(step: int, num_frames: int, batch_size: int, num_processes: int) -> dict:
+    """Map an optimization step to an `EpisodeAwareSampler` state for sample-exact resume.
+
+    Under accelerate's batch sharding, one step consumes `batch_size * num_processes` sampler
+    positions and each rank sees `ceil(ceil(num_frames / batch_size) / num_processes)` batches
+    per epoch (`even_batches` padding included). The start index provably stays below
+    `num_frames`; the `min` is defensive.
+
+    Assumptions (resume is only sample-exact when they hold):
+        - `num_processes` and `batch_size` match the run that wrote the checkpoint. Both scale how
+          many positions a step consumes, so the epoch/offset are wrong if either changed. The
+          caller passes the checkpoint's `num_processes` and `batch_size` and warns on a mismatch.
+        - accelerate uses `even_batches=True` (its default). The `ceil(... / num_processes)` term
+          mirrors that padding; with `even_batches=False` the per-epoch batch count differs and
+          the boundary is off.
+    """
+    batches_per_epoch = math.ceil(math.ceil(num_frames / batch_size) / num_processes)
+    epoch, batches_into_epoch = divmod(step, batches_per_epoch)
+    start_index = min(batches_into_epoch * batch_size * num_processes, num_frames)
+    return {"epoch": epoch, "start_index": start_index}

src/lerobot/scripts/lerobot_train.py

@@ -36,6 +36,8 @@ from tqdm import tqdm
 from lerobot.common.train_utils import (
     get_step_checkpoint_dir,
     get_step_identifier,
+    load_training_batch_size,
+    load_training_num_processes,
     load_training_state,
     save_checkpoint,
     update_last_checkpoint,
@@ -43,7 +45,7 @@ from lerobot.common.train_utils import (
 from lerobot.common.wandb_utils import WandBLogger
 from lerobot.configs import parser
 from lerobot.configs.train import TrainPipelineConfig
-from lerobot.datasets import EpisodeAwareSampler, make_dataset
+from lerobot.datasets import EpisodeAwareSampler, compute_sampler_state, make_dataset
 from lerobot.envs import close_envs, make_env, make_env_pre_post_processors
 from lerobot.optim.factory import make_optimizer_and_scheduler
 from lerobot.policies import PreTrainedPolicy, make_policy, make_pre_post_processors
@@ -232,14 +234,16 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
         torch.backends.cudnn.benchmark = True
     torch.backends.cuda.matmul.allow_tf32 = True
 
-    # Dataset loading synchronization: main process downloads first to avoid race conditions
+    # Dataset loading synchronization: the global main process downloads once to the shared
+    # dataset root, then a barrier lets every other rank read the already-populated copy.
+    # LeRobotDataset skips its snapshot_download when try_load() succeeds, so no rank re-downloads.
     if is_main_process:
         logging.info("Creating dataset")
         dataset = make_dataset(cfg)
 
     accelerator.wait_for_everyone()
 
-    # Now all other processes can safely load the dataset
+    # Other ranks read from the shared copy populated by the main process.
     if not is_main_process:
         dataset = make_dataset(cfg)
 
@@ -384,15 +388,47 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
         logging.info(f"{num_total_params=} ({format_big_number(num_total_params)})")
 
     # create dataloader for offline training
-    if hasattr(active_cfg, "drop_n_last_frames"):
+    if not cfg.dataset.streaming:
+        # All non-streaming (map-style) datasets use EpisodeAwareSampler.
+        # The order is a pure function of (seed, epoch), so every rank independently produces the
+        # same permutation. accelerate then shards it disjointly across ranks via BatchSamplerShard
+        # without needing a `generator` attribute to synchronize an RNG, and resume is sample-exact.
         shuffle = False
         sampler = EpisodeAwareSampler(
             dataset.meta.episodes["dataset_from_index"],
             dataset.meta.episodes["dataset_to_index"],
             episode_indices_to_use=dataset.episodes,
-            drop_n_last_frames=active_cfg.drop_n_last_frames,
+            drop_n_last_frames=getattr(active_cfg, "drop_n_last_frames", 0),
             shuffle=True,
+            seed=cfg.seed if cfg.seed is not None else 0,
         )
+        if cfg.resume and step > 0:
+            # The resume offset depends on the (num_processes, batch_size) that produced `step`, so
+            # use the values recorded in the checkpoint (falling back to the current ones for older
+            # ckpts that did not store them).
+            saved_num_processes = load_training_num_processes(cfg.checkpoint_path)
+            saved_batch_size = load_training_batch_size(cfg.checkpoint_path)
+            ckpt_num_processes = saved_num_processes or accelerator.num_processes
+            ckpt_batch_size = saved_batch_size or cfg.batch_size
+            if is_main_process and saved_num_processes not in (None, accelerator.num_processes):
+                logging.warning(
+                    f"Resuming with num_processes={accelerator.num_processes} but the checkpoint was "
+                    f"written with num_processes={saved_num_processes}. The data order resumes at the "
+                    "right epoch/offset, but per-rank sample-exactness requires the same world size."
+                )
+            if is_main_process and saved_batch_size not in (None, cfg.batch_size):
+                logging.warning(
+                    f"Resuming with batch_size={cfg.batch_size} but the checkpoint was written with "
+                    f"batch_size={saved_batch_size}. The data order resumes at the right epoch/offset, "
+                    "but per-rank sample-exactness requires the same batch size."
+                )
+            sampler_state = compute_sampler_state(step, len(sampler), ckpt_batch_size, ckpt_num_processes)
+            sampler.load_state_dict(sampler_state)
+            if is_main_process:
+                logging.info(
+                    f"Resuming data order at epoch {sampler_state['epoch']}, "
+                    f"sample {sampler_state['start_index']}"
+                )
     else:
         shuffle = True
         sampler = None
@@ -511,6 +547,8 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
                     scheduler=lr_scheduler,
                     preprocessor=preprocessor,
                     postprocessor=postprocessor,
+                    num_processes=accelerator.num_processes,
+                    batch_size=cfg.batch_size,
                 )
                 update_last_checkpoint(checkpoint_dir)
                 if wandb_logger:

tests/datasets/test_sampler.py

@@ -114,6 +114,19 @@ def test_shuffle():
     assert set(sampler) == {0, 1, 2, 3, 4, 5}
 
 
+def test_shuffle_is_reproducible_across_instances():
+    # The order is a pure function of (seed, epoch), so two fresh samplers (e.g. two ranks)
+    # produce the same permutation without any generator synchronization.
+    sampler_a = EpisodeAwareSampler([0], [6], shuffle=True, seed=42)
+    sampler_b = EpisodeAwareSampler([0], [6], shuffle=True, seed=42)
+    epoch_0 = list(sampler_a)
+    assert list(sampler_b) == epoch_0
+    # Desyncing the global RNG must not affect the permutation.
+    sampler_c = EpisodeAwareSampler([0], [6], shuffle=True, seed=42)
+    torch.randperm(1000)  # consume global RNG, as rank-asymmetric code (e.g. eval) would
+    assert list(sampler_c) == epoch_0
+
+
 def test_negative_drop_first_frames_raises():
     with pytest.raises(ValueError, match="drop_n_first_frames must be >= 0"):
         EpisodeAwareSampler([0], [10], drop_n_first_frames=-1)
@@ -137,3 +150,87 @@ def test_partial_episode_drop_warns(caplog):
     # Episode 0 is skipped (1 frame, drop 1), Episode 1 keeps frames 2-5
     assert sampler.indices == [2, 3, 4, 5]
     assert "Episode 0" in caplog.text
+
+
+# --- seeded (seed, epoch) shuffling, resume, and state ---
+
+from lerobot.datasets.sampler import compute_sampler_state  # noqa: E402
+
+EPISODE_BOUNDS = ([0, 2, 3], [2, 3, 6])  # episodes of 2, 1 and 3 frames
+
+
+@pytest.mark.parametrize("num_frames", [1, 2, 3, 37, 64, 100])
+def test_deterministic_sampler_shuffle_is_permutation(num_frames):
+    for seed in (0, 1, 1234):
+        sampler = EpisodeAwareSampler([0], [num_frames], shuffle=True, seed=seed)
+        assert sorted(sampler) == list(range(num_frames))
+
+
+def test_deterministic_sampler_epochs_reproduce_and_differ():
+    sampler_a = EpisodeAwareSampler([0], [100], shuffle=True, seed=42)
+    sampler_b = EpisodeAwareSampler([0], [100], shuffle=True, seed=42)
+    epoch_0 = list(sampler_a)
+    assert list(sampler_b) == epoch_0  # same (seed, epoch) -> same order on any process
+    epoch_1 = list(sampler_a)  # __iter__ auto-advances the epoch
+    assert epoch_1 != epoch_0
+    assert sorted(epoch_1) == sorted(epoch_0)
+    sampler_a.set_epoch(0)
+    assert list(sampler_a) == epoch_0
+    assert list(EpisodeAwareSampler([0], [100], shuffle=True, seed=7)) != epoch_0
+
+
+def test_deterministic_sampler_resume_mid_epoch():
+    reference = EpisodeAwareSampler(*EPISODE_BOUNDS, shuffle=True, seed=42)
+    epoch_0 = list(reference)
+    epoch_1 = list(reference)
+    for start in (0, 1, 4, len(epoch_0)):
+        resumed = EpisodeAwareSampler(*EPISODE_BOUNDS, shuffle=True, seed=42)
+        resumed.load_state_dict({"epoch": 0, "start_index": start})
+        assert list(resumed) == epoch_0[start:]
+        # the resumed sampler continues into the same epoch 1 as the uninterrupted one
+        assert list(resumed) == epoch_1
+
+
+def test_deterministic_sampler_construction_stores_only_boundaries():
+    # Construction is O(num_episodes), not O(num_frames): a million-frame single episode
+    # instantiates from just its boundaries without materializing a per-frame index list.
+    num_frames = 1_000_000
+    sampler = EpisodeAwareSampler([0], [num_frames], shuffle=True, seed=0)
+    assert len(sampler) == num_frames
+    assert sampler._starts.shape == (1,) and sampler._cum_lengths.shape == (1,)
+
+
+def test_deterministic_sampler_resume_is_exact_at_scale():
+    # Seeded randperm makes resume sample-exact at non-trivial sizes: regenerating the epoch's
+    # permutation and slicing from the saved offset reproduces the remaining order exactly.
+    num_frames = 100_000
+    reference = EpisodeAwareSampler([0], [num_frames], shuffle=True, seed=0)
+    epoch_0 = list(reference)
+    assert sorted(epoch_0) == list(range(num_frames))
+    start = num_frames - 5
+    resumed = EpisodeAwareSampler([0], [num_frames], shuffle=True, seed=0)
+    resumed.load_state_dict({"epoch": 0, "start_index": start})
+    assert list(resumed) == epoch_0[start:]
+
+
+def test_compute_sampler_state():
+    # 100 frames, batch 10, 2 ranks -> 10 underlying batches, 5 per rank per epoch.
+    assert compute_sampler_state(step=0, num_frames=100, batch_size=10, num_processes=2) == {
+        "epoch": 0,
+        "start_index": 0,
+    }
+    # step 7 -> epoch 1, 2 per-rank batches in = 2 * 10 * 2 = 40 samples in
+    assert compute_sampler_state(step=7, num_frames=100, batch_size=10, num_processes=2) == {
+        "epoch": 1,
+        "start_index": 40,
+    }
+    # uneven epoch: 95 frames -> 10 underlying batches (last short), still 5 per rank
+    assert compute_sampler_state(step=12, num_frames=95, batch_size=10, num_processes=2) == {
+        "epoch": 2,
+        "start_index": 40,
+    }
+    # uneven sharding: 105 frames -> 11 underlying batches, 6 per rank (even_batches pads)
+    assert compute_sampler_state(step=11, num_frames=105, batch_size=10, num_processes=2) == {
+        "epoch": 1,
+        "start_index": 100,
+    }

tests/utils/test_train_utils.py

@@ -20,6 +20,8 @@ from unittest.mock import Mock, patch
 from lerobot.common.train_utils import (
     get_step_checkpoint_dir,
     get_step_identifier,
+    load_training_batch_size,
+    load_training_num_processes,
     load_training_state,
     load_training_step,
     save_checkpoint,
@@ -63,6 +65,28 @@ def test_load_training_step(tmp_path):
     assert loaded_step == step
 
 
+def test_save_training_state_records_num_processes(tmp_path, optimizer, scheduler):
+    save_training_state(tmp_path, 10, optimizer, scheduler, num_processes=4)
+    assert load_training_num_processes(tmp_path) == 4
+
+
+def test_load_training_num_processes_absent_returns_none(tmp_path, optimizer, scheduler):
+    # Checkpoints written before the world size was recorded must still load (back-compat).
+    save_training_state(tmp_path, 10, optimizer, scheduler)
+    assert load_training_num_processes(tmp_path) is None
+
+
+def test_save_training_state_records_batch_size(tmp_path, optimizer, scheduler):
+    save_training_state(tmp_path, 10, optimizer, scheduler, batch_size=32)
+    assert load_training_batch_size(tmp_path) == 32
+
+
+def test_load_training_batch_size_absent_returns_none(tmp_path, optimizer, scheduler):
+    # Checkpoints written before the batch size was recorded must still load (back-compat).
+    save_training_state(tmp_path, 10, optimizer, scheduler)
+    assert load_training_batch_size(tmp_path) is None
+
+
 def test_update_last_checkpoint(tmp_path):
     checkpoint = tmp_path / "0005"
     checkpoint.mkdir()